An unusual waveform detection circuit, one of defect detecting circuits in a conventional optical disc, is illustrated in FIG. 12, together with a processing circuit system of a playback signal of an information playback apparatus.
In FIG. 12, playback signal processing circuit 100 in an optical disc includes optical pickup 102 for reading a recorded signal from recording medium 101, preamplifier 103 for pre-amplifying an output signal from optical pickup 102, variable gain amplifier 104 for further amplifying and adjusting an output signal from preamplifier 103, analog filter 105 for filtering an output signal from variable gain amplifier 104, analog/digital (A/D) converter 106 for converting an output signal from analog filter 105 from analog to digital, digital filter 107 for filtering an output signal from A/D converter 106, decoder 108 for decoding an output signal from digital filter 107 and outputting playback data, and analog-type unusual waveform detection circuit 109 for detecting unusualness of an output signal waveform from analog filter 105.
Unusual waveform detection circuit 109, as shown in FIG. 13 and FIG. 14, has slicer 109A for binarizing an analog playback signal with reference to a predetermined threshold, and mono-multi circuit 109B for producing an unusual waveform detection signal based on the binarized signal.
An operation of these elements is schematically described hereinafter.
Generally, when information is stored in an optical disc such as a CD (compact disc) or DVD (digital versatile disc) or the information is played back from the optical disc, the following procedure is used. Analog information such as video or music is encoded to digital data, then modulated, and written into an information-by-information pit on recording medium 101 such as the optical disc.
In playing back the data, the pit that is recording the data is firstly scanned with a laser. Optical pickup 102 takes in laser reflected beam from recording medium 101 and converts intensity of the reflected beam to an electric signal to provide an analog playback signal. The analog playback signal provided by optical pickup 102 is pre-amplified by preamplifier 103, and then adjusted in amplitude by variable gain amplifier 104.
Then, analog filter 105 boosts a high frequency component of the playback signal for waveform equalization. A/D converter 106 digitizes the analog playback signal having passed through analog filter 105. A digital filtering process is performed after this process by A/D converter 106. In other words, the digitized data is corrected by digital filter 107 and decoded by decoder 108, and decoder 108 outputs the binary digital playback data that is stored in the pit.
In analog-type unusual waveform detection circuit 109, as shown in FIG. 13 and FIG. 14, slicer 109A binarizes voltage of the analog playback signal having passed through analog filter 105 with reference to the preset threshold voltage (slicer output in FIG. 14). Mono-multi circuit 109B then receives the binarized signal and produces an unusual waveform detection signal.
In decoding the data recorded in the recording medium such as the optical disc, waveform unusualness of the playback signal due to a defect of the recording pit in the optical disc causes wrong operation of a subsequent signal processing circuit. Therefore, the unusual waveform signal must be detected in a recording and reproducing system. The analog-type unusual waveform detection circuit for detecting an unusual waveform signal using an analog playback signal is presently wide used. However, the analog type detecting circuit cannot sometimes easily and accurately detect the unusual waveform signal because the circuit is adversely affected by a broadband analog playback signal and noise. The analog type detecting circuit also requires an external component and must be large in scale.